During 1986, the latest year for which data have been published, 57,036 deep wells were drilled in the United States and Canada in the search for oil and gas, and about 20,000 were drilled in the rest of the world. T he wells in North America penetrated some 46,328 miles into the Earth, providing access to an immense amount of otherwise invisible geology. Wells were also drilled for other reasons, such as the search for water, economic mineral resources, and even scientific information. Considering only the holes drilled for oil and gas, virtually every one has been logged with one or more geophysical surveys. In the last 40 years, several million such geophysical surveys have been run and are now preserved in various data libraries. This data base of geophysical surveys run in boreholes is perhaps the largest data base we have in the Earth sciences, and to date, it has been relatively little utilized for any purpose other than evaluating possible hydrocarbon content in suspected reservoir rocks. In the past decade there has been a growing interest in making more use of this data base, which may well explain the appearance of at least a half dozen books on geophysical well surveying over the last several years.